This invention relates to the sound amplification arts and to their application in the amelioration of auditory deficiencies resulting from damage to the sensori-neural structure of the human ear. It relates particularly to apparatus for correcting deficiencies in a person's ability to perceive and to comprehend spoken language.
Sensori-neural hearing loss is generally considered to be the most prevalent type of auditory handicap found in the United States as well as in other civilized cultures. It constitutes a significant barrier to adequate communication in 5% to 10% of the total United States population and in more than 50% of the population over 60 years of age. Furthermore, these proportions are expected to increase in conjunction with ongoing increases in ambient noise levels and life expectancy in our society.
Sensori-neural impairment may result from any one or more of a number of causes including, but not limited to, genetic and congenital factors, viral diseases, specific toxic agents, circulatory disturbances, specific physical trauma and excessive exposure to noise. Irrespective of the primary cause, however, sensory cells within the organ of hearing or their associated neural units suffer some degree of damage and are rendered partially or totally incapable of fulfilling their respective roles in the processing of auditory information. This form of damage cannot be repaired by means of currently known medical or surgical techniques, and the probability of discovery of effective techniques, within the forseeable future appears rather remote. Thus, in virtually all cases of sensori-neural hearing loss, amplification of incoming sounds represents the only possible means for restoring adequate hearing ability.
Hearing loss resulting from sensori-neural damage is usually irregular with respect to frequency, being selectively greater for particular portions of the audible frequency range. The ability to hear sounds in the range above 1000 Hz is often affected more than the hearing of sounds below 1000 Hz, although this is by no means a universal observation. The ultimate consequency of irregular hearing acuity for various portions of the audio frequency spectrum is distortion in the perception of complex sounds, i.e., sounds composed of a number of different frequencies.
A certain amount of distortion in complex sounds may be tolerable, but current information does not permit precise specification of the maximum amount of each type of distortion which may exist without interfering materially with accurate sound recognition. Many gross sounds, for example, do not demand a great deal of analytic power in the auditory system, so even a rather severely impaired system may function adequately in the interpretation of such sounds.
In audiologic parlance, the term "discrimination" denotes the capacity of the ear to analyze incoming acoustic patterns and interpret them appropriately. Analytic power may fail at any of several stages in the auditory process, commonly in the organ of hearing or first order neurons due to damage to these structures. Since the ear may be required to perform many degrees of discrimination, varying from extremely coarse to extremely fine, its analytic power may be measured through the use of tests which demand auditory discriminations of progressive difficulty until failure occurs.
Among the most difficult discriminations required of the human ear are those necessary for accurate interpretation of speech, particularly speech in the presence of noise. Tests of speech discrimination are commonly employed, therefore, to derive a realistic estimate of a person's everyday functional adequacy in hearing.
Each of the phonic units of a spoken word is a complex sound, composed of several frequencies clustered in a more-or-less definable range. When the acuity of the ear has been selectively impaired in a specific frequency range, speech sounds or their components falling in that range may be heard at a reduced intensity or not at all. Impairment in several frequency ranges compounds the difficulty and is probably responsible in large measure for the primary complaint of the individual with sensori-neural hearing loss that he can hear a speaker's voice but cannot understand what is said. The mechanism for inhibiting such understanding may be the nonlinear responses that result in intermodulation products and harmonics which could cause interference with the desired spectral components of speech.
On the basis of the foregoing information, it would seem quite reasonable to deal with sensori-neural hearing loss by selective spectrum amplification; that is, providing amplification only in those frequency ranges or bands in which acuity is deficient, and only in the amount of the deficiency. Thus, the ultimate value of selective spectrum amplification rests on the application of appropriate methods for measuring the degree of auditory deficiency as a function of various frequency bands, and also on the construction of a wearable device which is fully capable of producing amplification to compensate for the measured deficiencies. Because of existing inadequacies in both respects, the principle of selective amplification has fallen into disrepute, for the hearing aid industry has adopted the pure tone (single frequency) threshold audiogram as the criterion measurement and has produced hearing aids with inadequate capabilities for providing proper acoustic output at each portion of the audio band.
The threshold audiogram curve represents an individual's measured absolute auditory threshold for a series of pure frequency tones, usually in the range of 250 Hz to 8000 Hz sampled at octave intervals on the assumption that intra-octave tone thresholds follow the general audiogram contour. However, it is demonstrable that fairly marked departures from this overall pattern may exist at intermediate frequencies, i.e., frequencies between pure tones one octave apart. In fact, careful consideration of the types of measurements which are genuinely helpful in guiding the design of particular hearing aid features suggests that the pure tone threshold curve is virtually useless for several reasons:
A. under everyday circumstances, individuals react only to supra-threshold sounds, as these are sounds of primary significance. For practical purposes, threshold sounds remain unnoticed. PA1 B. the contour of an individual's threshold curve is observably different from the contour of his supra-threshold equal loudness curves or comfortable listening level curves. PA1 C. an individual's recognition of complex phonic units or their combination into spoken words is essentially unrelated to his acuity for individual pure tones.
Control of acoustic output in current hearing aids is ordinarily achieved through manipulation of frequency response, which refers to the acoustic output of a sound transmission system at each of the frequencies within its pass band when the input level is maintained constant for all frequencies. A graphic representation of a system's frequency response is referred to as a response characteristic, curve or contour. Manufacturers commonly claim that they are able to build hearing aids to yield any required frequency response; but this does not appear to be the case in practice because there are definite limitations on the band widths and response curves available in present day aids. In practice, manufacturers use combinations of components which produce a limited choice of response patterns and simply select one which most closely corresponds to the criterion, which, as mentioned earlier, usually is a threshold audiogram curve.
One additional comment is relevant as a preface to the innovative concepts to which the present invention is particularly addressed. It is generally recognized that the ear with sensori-neural hearing loss is excessively susceptible to overloading, which is to say that, although it may be relatively insensitive to sounds of low or moderate intensity, it is hypersensitive to sounds of higher intensity (i.e., nonlinear response characteristics). This condition restricts the useful operating range of the ear, referred to as the dynamic range; that is, the decibel difference between the lowest intensity at which a sound is reliably detected (absolute threshold) and the upper limit of comfortable loudness for that sound (discomfort threshold).
Whereas, the dynamic range of the normal ear is of the order of 100 dB, the range of a sensori-neurally impaired ear may be as little as 10 or 15 DB, generally over a limited frequency spectrum range. Thus, for an impaired ear to function with any degree of adequacy, the full intensity range of thet outside acoustic world must be restricted in some way to fit through an abnormally small sound window and such restriction must cause minimal intermodulation products, harmonics, and so forth which would result in distortion. Without such restriction, the ear is readily overloaded, leading to psychologic or physical annoyance and distortion of incoming acoustic patterns.
The consequences of overloading have been appreciated for many years, and output compression devices are widely used in today's hearing aids. Without exception, however, these devices operate on a broad frequency band, so that when any frequency component of a signal reaches a predetermined critical level, the entire pass band of the hearing aid is compressed. Consequently, the components which are not at a critical intensity are needlessly attenuated.
Our evaluation of relevant factors have led to the evolution of several innovative concepts providing improved methods and apparatus for measuring and defining auditory deficiencies in terms of a prescription for compenstory amplification, but then the remaining difficulty is the proper adjustment of the prosthetic device itself in order to assure that the prescribed compensatory amplification spectrum is operatively provided in the prosthetic device.
Modern wearable hearing aids vary considerably in circuitry and in the degree to which adjustments can be made in operating characteristics. Some have volume controls, maximum power output controls, some have no filter, others may have one or more filters, which provide frequency bands which may or may not be adjacent. In fact, in some hearing aids having multiple filters, one filter may provide a band which is completely or almost completely within the band of another filter. Even though gain controls may be provided for specific frequency bands, such devices are difficult to properly calibrate, particularly those aids worn entirely in or about the ear, they being quite small. Generally, no calibration indicia are provided for such devices, the adjusting screws are very small and are usually located inside a cover which must be removed in order to obtain access thereto. Adjusting a hearing aid while it is fitted to the patient's ear is considered a very unsatisfactory procedure as the very act of adjusting the tiny screws creates noises which are annoying to the patient. In addition to those problems, it is highly desirable to use unskilled or semiskilled labor for fitting hearing aids, and therefore a sufficiently simple and foolproof method and apparatus for incorporating the hearing profile prescription into the aid would be of considerable value.